Enhancement of mechanical properties by repeated heat treatment in reduced activation ferritic/martensitic steel with Ta and Ti

Abstract

The effects of heat treatment conditions on the microstructures and mechanical properties of Ta and Ti added reduced activation ferritic/martensitic (TaTi-RAFM) steel were investigated in this study. The authors attempted to control the precipitation of MX particles and M23C6 carbide, which critically affects the mechanical properties of RAFM steels, by repetition of normalizing and tempering, and the resultant change of microstructure and mechanical properties was examined. Double-normalizing was effective for improving the ductile-brittle transition temperature (DBTT) in TaTi-RAFM steel. Moreover, the DBTT was further reduced by double-tempering in double-normalized TaTi-RAFM steel. This improvement of the DBTT by double-normalizing or double-normalizing and double-tempering results from an increased fraction of MX precipitate and refinement of M23C6 carbide, which is confirmed by a quantitative analysis based on more than ten SEM micrographs with high magnification. The tendency of DBTTs in variously heat-treated specimens was consistent with the tendency of fracture strengths calculated using the modified Griffith model with maximum size of M23C6 carbide, which is vulnerable to fracture.